Q1(g). Packing fraction

Answer in brief

In nuclear chemistry, packing fraction is the fractional difference between isotopic mass and mass number: f=(M-A)/A, often tabulated as 10⁴f.

Key points

• M is the numerical isotopic atomic mass in unified atomic mass units and A is the integer mass number.
• Positive f means M>A; negative f means M<A. It measures mass excess per nucleon relative to the mass-number reference.
• It is not the same as mass defect relative to separated protons and neutrons; binding energy is calculated from that mass defect.
• In solid-state chemistry the same phrase denotes the fraction of a unit cell occupied by hard spheres: APF=n(4π r³/3)/a³.